The present invention relates to a charge pump, and more particularly to a charge pump for use in a PLL (Phase Locked Loop).
The PLL has recently been mounted not only in microprocessors but also in high-speed memories, and is considered more and more important. For particulars of the PLL, see, for example, Deog-Kyoon Jeong et al., xe2x80x9cDesign of PLL-Based Clock Generation Circuitxe2x80x9d IEEE Journal of Solid-State Circuits, Vol. SC-22, No. 2, pp. 255-261, April 1987.
The PLL generally has a charge pump, which will be described.
FIG. 1 is a circuit diagram, showing an example of a charge pump. This charge pump operates as below, supposing that a low pass filter (LPF) is connected to the charging terminal of the charge pump.
A p-channel MOS transistor (hereinafter referred to simply as a xe2x80x9cpMOS transistorxe2x80x9d) P100 is driven by a charge signal as shown in FIG. 2. Then, the charge pump charges the LPF with electricity corresponding to the pulse width of the charge signal. Further, an n-channel MOS transistor (hereinafter referred to simply as an xe2x80x9cnMOS transistorxe2x80x9d) N100 is driven by a discharge signal as shown in FIG. 2. Then, the charge pump discharges, from the LPF, electricity corresponding to the pulse width of the discharge signal.
The relationship between the total charge and the pulse width is controlled depending upon the sizes of current-limiting transistors P102 and N102 which are located at the source-sides of the pMOS transistor P100 and the nMOS transistor N100, or upon the input gate voltage (Vref).
Moreover, an nMOS transistor N104 and a pMOS transistor P104 are controlled by an enable signal and an enable bar signal which is the inverted signal of the enable signal, respectively. The nMOS transistor N104 and the pMOS transistor P104 serve as filters for interrupting a pass current when the charge pump is in a standby state, and removing switching noise while the charge pump operates.
As is shown in FIG. 1, there exist PN junctions at the drain-sides of the limiter transistors P102 and N102 of the charge pump, and junction capacitors C102 and C104 exist at the PN junctions. Accordingly, at the start of the charge pump, the current-limiting transistor P102 or N102 operates for the first time after the junction capacitor C102 or C104 is charged or discharged. This being so, electricity discharged from the junction capacitor C102 or that to be accumulated into the junction capacitor C104 (the electricity will be referred to as a xe2x80x9cto-be-offset (or offset) chargexe2x80x9d) is added to electricity to be accumulated into or discharged from the low pass filter.
The offset charges will be described in detail. A case where charging is performed at the pMOS transistor side of the charge pump will be examined. If no junction capacitor C102 exists, the potential of the junction between the limiter transistor P102 and pMOS transistor P100 becomes less than a power voltage VDD and reaches the operation voltage of the limiter transistor P102 at the moment the charge signal level has changed to xe2x80x9cL (Low)xe2x80x9d. The current which flows when the operation voltage has been reached is an ideal average current.
On the other hand, where there exists the junction capacitor C102 at the junction between the limiter transistor P102 and the pMOS transistor P100 as shown in FIG. 1, the potential of the junction has a value between the power voltage VDD and an operation voltage V1 until electricity flows from the junction capacitor C102 to the LPF via the pMOS transistor P100, even if the charge signal has shifted to xe2x80x9cLxe2x80x9d, as is shown in FIG. 3. At this time, even if the pMOS transistor P100 operates in the pentode area, a current which includes an overshooting portion as shown in FIG. 3 and hence is greater than the ideal average current flows into the LPF. This extra current is the cause of the offset, and the hatched overshooting portion in FIG. 3 corresponds to the offset current.
When the pulse width of each of the discharge and charge signals is sufficiently large, the effective average current (=(total charge)/(pulse width)) is kept substantially constant even if a slight current is discharged from the junction capacitor C102 or charged into the junction capacitor C104. However, where the pulse width is small, only a slight offset current will increase the effective average current. An increase in the effective average current requires an increase in the capacity of the LPF in order to keep the entire PLL system stable, which will inevitably increase the lockup time or layout area.
It is the object of the invention to provide a charge pump which is substantially free from the influence, upon a total charge, of a junction capacitor existing at a PN junction, even if the pulse width of a driving signal used in the pump is small, and hence in which pump the effective average current shows only a little dependency upon the pulse width during the charging or discharging operation of the pump.
According to a first aspect of the invention, there is provided a charge pump comprising: a first sub charge pump having a plurality of transistors for performing charging and discharging operations; and a second sub charge pump having a charging/discharging terminal common to that of the first sub charge pump, the second sub charge pump discharging electricity which is charged via the charging/discharging terminal when the first sub charge pump has performed a charging operation using electricity accumulated in a junction capacitor at a PN junction which exists between the transistors of the first sub charge pump, the second sub charge pump charging electricity via the charging/discharging terminal when the first sub charge pump has performed a discharging operation to charge the junction capacitor.
The charge pump constructed as above can substantially cancel electricity which is discharged from the junction capacitor existing at the PN junction, and is used as a charging current at the time of charging, and also can substantially cancel electricity which is accumulated in the junction capacitor and used as a discharging current at the time of discharging. As a result, even if the pulse width of a driving signal is small, the influence of the junction capacitor upon the total charge can be minimized, thereby suppressing an increase in average current when the charge pump performs charging and discharging operations, and reducing the dependency of the effective average current upon the pulse width.
According to a second aspect of the invention, there is provided a charge pump comprising: a first sub charge pump having a plurality of transistors for performing charging and discharging operations; and a second sub charge pump having a charging/discharging terminal common to that of the first sub charge pump, the second sub charge pump performing a charging operation for a time period shorter by a predetermined time period than a charging time period for which the first sub charge pump performs a charging operation, the second sub charge pump performing a discharging operation for a time period shorter by a predetermined time period than a discharging time period for which the first sub charge pump performs a discharging operation.
In the charge pump constructed as above, the first sub charge pump performs a charging operation for the charging time period, and the second sub charge pump performs a charging operation for a time period shorter by a predetermined time period than the charging time period of the first charge pump. On the other hand, the first sub charge pump performs a discharging operation for the discharging time period, and the second sub charge pump performs a discharging operation for a time period shorter by a predetermined time period than the discharging time period of the first charge pump. As a result, variations in average current during charging and discharging of the charge pump can be minimized, thereby reducing the dependency of the effective average current upon the pulse width.
According to a third aspect of the invention, there is provided a charge pump comprising: a plurality of first sub charge pumps having a plurality of transistors for performing charging and discharging operations; and a plurality of second sub charge pumps having a charging/discharging terminal common to that of the first sub charge pumps, the second sub charge pumps discharging electricity which is charged via the charging/discharging terminal when the first sub charge pumps have performed a charging operation using electricity accumulated in junction capacitors at PN junctions which exist between the transistors of the first sub charge pumps, the second sub charge pumps charging electricity via the charging/discharging terminal when the first sub charge pumps have performed a discharging operation to charge the junction capacitors.
The charge pump constructed as above can substantially cancel electricity which is discharged from the junction capacitor existing at each PN junction, and is used as a charging current at the time of charging, and also can substantially cancel electricity which is accumulated in the junction capacitor and used as a discharging current at the time of discharging. As a result, even if the pulse width of a driving signal is small, the influence of the junction capacitor upon the total charge can be minimized, thereby suppressing an increase in average current when the charge pump performs charging and discharging operations, and reducing the dependency of the effective average current upon the pulse width.
According to a fourth aspect of the invention, there is provided a charge pump comprising: a plurality of first sub charge pumps having a plurality of transistors for performing charging and discharging operations; and a plurality of second sub charge pumps having a charging/discharging terminal common to that of the first sub charge pumps, the second sub charge pumps performing a charging operation for a time period shorter by a predetermined time period than a charging time period for which the first sub charge pumps perform a charging operation, the second sub charge pumps performing a discharging operation for a time period shorter by a predetermined time period than a discharging time period for which the first sub charge pumps perform a discharging operation.
In the charge pump constructed as above,:the first sub charge pumps perform a charging operation for the charging time period, and the second sub charge pumps perform a charging operation for a time period shorter by a predetermined time period than the charging time period of the first charge pumps. On the other hand, the first sub charge pumps perform a discharging operation for the discharging time period, and the second sub charge pumps perform a discharging operation for a time period shorter by a predetermined time period than the discharging time period of the first charge pumps. As a result, variations in average current during charging and discharging of the charge pump can be minimized, thereby reducing the dependency of the effective average current upon the pulse width.
According to a fifth aspect of the invention, there is provided a charge pump comprising: a first sub charge pump having a plurality of transistors for performing a charging operation in response to a charging signal and a discharging operation in response to a discharging signal; and a second sub charge pump having a charging/discharging terminal common to that of the first sub charge pump, the second sub charge pump performing a discharging operation in response to a pulse signal generated from the charging signal, when the first sub charge pump has performed a charging operation in response to the charging signal using electricity accumulated in a junction capacitor at a PN junction which exists between the transistors of the first sub charge pump, the second sub charge pump performing a charging operation in response to a pulse signal generated from the discharging signal, when the first sub charge pump has performed a discharging operation in response to the discharging signal to charge the junction capacitor.
The charge pump constructed as above can substantially cancel electricity which is discharged from the junction capacitor existing at the PN junction, and is used as a charging current at the time of charging, and also can substantially cancel electricity which is accumulated in the junction capacitor and used as a discharging current at the time of discharging. As a result, even if the pulse width of a driving signal is small, the influence of the junction capacitor upon the total charge can be minimized, thereby suppressing an increase in average current when the charge pump performs charging and discharging operations, and reducing the dependency of the effective average current upon the pulse width.
According to a sixth aspect of the invention, there is provided a charge pump comprising: a first sub charge pump having a plurality of transistors for performing a charging operation in response to a charging signal of a first pulse width, and performing a discharging operation in response to a discharging signal of a second pulse width; and a second sub charge pump having a charging/discharging terminal common to that of the first sub charge pump, the second sub charge pump performing a charging operation in response to a pulse signal of a third pulse width shorter by a predetermined width than the first pulse width when the first sub charge pump performs the charging operation in response to the charging signal of the first pulse width, the second sub charge pump performing a discharging operation in response to a pulse signal of a fourth pulse width shorter by a predetermined width than the second pulse width when the first sub charge pump performs the charging operation in response to the charging signal of the second pulse width.
In the charge pump constructed as above, the first sub charge pump performs a charging operation in response to the charging signal of the first pulse width, and the second sub charge pump performs a charging operation in response to the pulse signal of the third pulse width smaller by the predetermined width than the first pulse width. On the other hand, the first sub charge pump performs a discharging operation in response to the discharging signal of the second pulse width, and the second sub charge pump performs a discharging operation in response to the pulse signal of the fourth pulse width smaller by the predetermined width than the second pulse width. As a result, variations in average current during charging and discharging of the charge pump can be minimized, thereby reducing the dependency of the effective average current upon each pulse width.
According to a seventh aspect of the invention, there is provided a charge pump comprising: a first sub charge pump having a charging section wherein the source of a first p-channel MOS transistor is connected to the drain of a second p-channel MOS transistor, the drain of the first p-channel MOS transistor is connected to the source of a third p-channel MOS transistor, the source of the second p-channel MOS transistor is connected to a power voltage, and the drain of the third p-channel MOS transistor is connected to a charging/discharging terminal, the first sub charge pump also having a discharging section wherein the drain of a first n-channel MOS transistor is connected to the source of a second n-channel MOS transistor, the source of the first n-channel MOS transistor is connected to the drain of a third n-channel MOS transistor, the drain of the second n-channel MOS transistor is connected to a ground voltage, and the source of the third n-channel MOS transistor is connected to the charging/discharging terminal; and a second sub charge pump having a charging section wherein the source of a fourth p-channel MOS transistor is connected to the drain of a fifth p-channel MOS transistor, the drain of the fourth p-channel MOS transistor is connected to the source of a sixth p-channel MOS transistor, the source of the fifth p-channel MOS transistor is connected to the power voltage, and the drain of the sixth p-channel MOS transistor is connected to the charging/discharging terminal, the second sub charge pump also having a discharging section wherein the drain of a fourth n-channel MOS transistor is connected to the source of a fifth n-channel MOS transistor, the source of the fourth n-channel MOS transistor is connected to the drain of a sixth n-channel MOS transistor, the drain of the fifth n-channel MOS transistor is connected to the ground voltage, and the source of the sixth n-channel MOS transistor is connected to the charging/discharging terminal.
Even if the pulse width of a driving signal is small in the charge pump constructed as above, the influence of the junction capacitor upon the total charge can be minimized, thereby suppressing an increase in average current when the charge pump performs charging and discharging operations, and reducing the dependency of the effective average current upon the pulse width.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.